Programmed Cell Death in Plants.

Abstract

There is increasing evidence for occurrence of programmed cell death (PCD) in plant development, plant-microbe interaction and cells under a variety of stresses. Recent studies on PCD in plants indicate that various features of apoptosis in mammals are shared with plant PCD: there is evidence for DNA fragmentation, oligonucleosomal DNA laddering, morphological changes in plant cells. These studies suggest that PCD plays an important role in the life of plants as in animals. Despite the wide occurrence of PCD in plants, signaling and components of the machinery for PCD are largely unknown. We recently identified the Rac family of the small GTP-binding protein as a key regulator of PCD in plants. Also, the analysis of lesion mimic mutants of rice indicates that some mutants have biochemical alterations in early steps of signaling in disease resistance. The major challenge in the study of plant PCD in the near future is the identification of signaling molecules and components of machinery involved in plant PCD. This will enable us to better understand this important cellular process of plants.ment of roots. Aerenchyma cells are formed by PCD to produce internal air spaces to facilitate efficient oxygen supply in the soil [8] . Another form of PCD that occurs in roots is found in root cap cells [9]. Root cap cells are produced from the meristem and sloughed from the root while new cells are continuously generated.Leaf: Leaf senescence is a well known example of cell death in plant development [10]. When old leaves senesce active transcription of new genes occurs, suggesting that senescence is an active process and some genes encode hydrolase such as proteases [10]. Although morphological changes of cells and DNA cleavage typically observed in cells undergoing PCD have not been clearly demonstrated in senescing leaves yet, a number of physiological studies strongly suggest that it involves PCD.Flower: Two examples of PCD have been recently demonstrated in flowers. In pea flowers, the carpel undergoes PCD when pollination is prevented by emasculation. This senescence of the carpel is stimulated by ethylene and inhibited by treatment with GA. In dying carpels, oligonucleosomal DNA fragmentaion has been observed [11]. Another example of PCD in flower development is elimination of sex organ in unisexual flowers. For instance, in maize stamen primordia abort early in development of female flowers [12].Many examples of PCD during normal development of plants described above strongly suggest that plants employ PCD to eliminate some cells or generate specialized cells such as TEs in development. However, at the moment little is known on molecular mechanisms regulating the PCD in plant development. Furthermore, whether there are multiple pathways for various PCDs observed during development remains to be studied in the future.